Poor describes having little of something. If you lack needed food and shelter, you're poor, and if you fall below a standard or don't even try, you're giving a poor performance. Empty pockets and empty efforts both mean poor.
Rich or wealthy often serve as opposites of poor. If you have material things, you're rich, and if you don't, you're poor. You can be full of knowledge but get poor grades if you're not studying, or you might be poor in terms of money but rich in friends and kindness. Poor can also be used as a noun, as in "Charity is aimed at helping the poor."